The present invention is directed generally to stainless steel strip casting. More particularly, the present invention is directed to Cr--Ni austenitic stainless steel strip and methods for producing the same employing strip casting.
One conventional method for producing stainless steel strip employs ingot casting. In ingot casting, molten steel is brought to a pouring stand in a steel ladle that usually holds about 200 tons of liquid steel. A series of ingot molds are positioned next to the pouring stand on top of ingot cars. Each ingot mold sits on a mold base which prevents leakage of the molten metal from the bottom of the ingot molds. The ingot molds are hollow tubes, rectangular in cross-section, made of cast iron. The ingot mold cross-sections are usually about 2 feet by 3 feet (0.6 m by 0.9 m), and the heights of the ingot molds are about 8 feet (2.4 m). The total weight of the steel required to fill a mold is usually about 10 to 20 tons.
Because the cross-section of an ingot mold is very large and the mold is not cooled by any means other than convection from the exterior surface of the mold, the cooling rate for most of the metal in the ingot mold is very low. Therefore, very large, branched crystals, called dendrites, form upon solidification, and hence, a very large grain size exists in the as-cast microstructure. The formation of large dendrites as a result of the slow cooling rate results in large volumes of segregated liquid solidifying around the branches of the dendrites (dendritic segregation), making the casting susceptible to internal cracking and voids.
After solidification and substantial cooling, the ingot is separated from the ingot mold and placed into a heating zone called a soaking pit where the ingot is reheated to a temperature above 2000.degree. F. (1093.degree. C.) to prepare the ingot for hot rolling. During an initial hot rolling operation called blooming, the ingot is reduced in width from about 2 feet (0.6 m) to approximately 7 to 10 inches (18 cm to 25.4 cm). The resulting semi-finished product is called a slab. The microstructure of the slab is very similar to that of the ingot and includes, predominantly, large dendrites and other large grains from the as-cast microstructure. Some of the as-cast microstructure was eliminated by the blooming operation, but not a significant portion because the amount of reduction resulting from the blooming operation is only about 2:1.
The slabs are transported to a hot strip mill where each slab is reheated to above 2000.degree. F. (1093.degree. C.) before undergoing additional hot rolling into a strip. After reheating, the slabs are sent through a series of roughing stands and then through a series of finishing stands in the hot strip mill to produce a strip having a thickness of about 0.1 inches (0.25 cm) which is then coiled. The hot working which the strip undergoes in the roughing and finishing stands usually is sufficient to break up almost all of the as-cast microstructure because the amount of reduction is about 100:1. The resulting grain size in the strip is usually rather large and can be controlled to some extent by controlling the cooling rate the strip undergoes after hot working in the hot strip mill and prior to coiling.
To produce a final product, the hot rolled strip is subjected to processing steps that include a first anneal, a first cold rolling, a second anneal, a second cold rolling, and a third anneal. The anneals may all be performed at temperatures in the range 900.degree.-1150.degree. C., for example, and the cold rolling steps may each produce a reduction of 50-65%. After the third anneal, typically a skin pass is performed on the strip. The skin pass is a cold rolling operation that produces small amounts of reduction (e.g., 0.5-2%) in the strip. The surface quality of the resulting strip, in terms of surface defects, roughness, reflectability and other conventional surface parameters, is generally very good. Ingot casting, however, is a relatively inefficient and uneconomical method for producing stainless steel strip.
Another method for producing stainless steel strip employs strip casting. In strip casting, molten metal is cast directly into a strip that is about 1.5-6 mm thick. One type of strip casting is twin roll casting. Twin roll casting is typically performed on apparatus comprising a pair of horizontally spaced rolls mounted for rotation in opposite rotational senses about respective horizontal axes. The two rolls define a horizontally disposed, vertically extending gap therebetween for receiving molten metal. The gap defined by the rolls tapers arcuately in a downward direction. Molten metal in the gap forms a pool. The rolls are cooled and, in turn, cool the molten metal as the molten metal from the pool descends through the gap. A solidified metal strip emerges from the gap.
Procedures employing strip casting, in contrast to procedures employing ingot casting, do not employ hot rolling. Consequently, the resulting Cr--Ni austenitic stainless steel strip generally has a typical as-cast microstructure (i.e., a microstructure having detrimental amounts of dendrites and delta ferrite). The subsequent processing steps performed on the as-cast strip in conventional processes that employ strip casting are similar to those steps performed after the hot rolling step in processes that employ ingot casting. These steps include a first anneal followed by a first cold rolling, a second anneal followed by a second cold rolling, a third anneal and, typically, a skin pass.
Strip casting is more efficient and economical than ingot casting. However, the stainless steel final product of processes employing strip casting and conventional annealing and cold rolling practices, generally has a surface quality inferior to that of stainless steel strip produced by processes employing ingot casting and hot rolling followed by annealing and cold rolling. The inferior surface quality includes less reflectability, more roughness and less homogeneity. Microstructural factors contributing to the inferior surface quality include residual or inherited dendritic structure at the surface, chemical non-homogeneity at the surface, crystallographic non-homogeneity at the surface (i.e., bands with different crystal orientation or crystalline texture at the surface), and residual delta ferrite. Chemical and crystallographic non-homogeneity are caused, in part, by the dendritic segregation which occurred during strip casting.
The crystal structure of delta ferrite (body-centered cubic) is different than the crystal structure of austenitic (face-centered cubic), the difference contributing to the non-homogeneous surface appearance. The body-centered cubic delta ferrite has a different deformability than the austenitic, leaving a "streaked" surface appearance after cold rolling.
In procedures employing ingot casting, the subsequent hot rolling of the ingot cast steel breaks down the as-cast microstructure, causing dendritic homogenization and delta ferrite dissolution. Homogenization is mainly a diffusion-based process in which the dendrites, which are austenitic in the as-cast orientation, become austenitic that is no longer in the as-cast orientation. More particularly, during homogenization, the branches of the dendrites become smaller and more similar in chemical composition to the surrounding metal. Delta ferrite dissolution is the conversion of delta ferrite to austenite. Thus, very little or no dendritic structure and very little delta ferrite is present in the microstructure of stainless steel strip which has been ingot cast and then hot rolled. In contrast, in procedures employing strip casting, there is no hot rolling step, and the resulting as-cast strip generally has an as-cast microstructure including detrimental amounts of dendritic structure and delta ferrite.
Stainless steel strip made by processes employing strip casting followed by conventional annealing and cold rolling practices has relatively poor surface quality. The poor surface quality of such strip, compared to strip produced by processes employing ingot casting, has been attributed to the presence of detrimental amounts of dendritic structure and delta ferrite in the final product. In contrast, a desired, final product microstructure generally comprises a relatively low amount of delta ferrite and comprises essentially austenite, with very little or none of the austenite being dendritic. A desirable, final product microstructure has an amount of delta ferrite near the equilibrium amount which, in an austenitic stainless steel, is about 3-5% by volume.
To reduce the amount of delta ferrite in the strip cast product, attempts have been made to cool the strip quickly, immediately after casting, down to about 1100.degree. C. Once the strip has cooled to about 1100.degree. C., the cooling rate of the strip is drastically reduced to allow solid state diffusion to transform most of the remaining delta ferrite into austenite. Such fast cooling, however, risks crackling the surface of the strip if the cooling is not performed very carefully.
As described above, conventional cold rolling practice following the strip casting step typically includes three anneals, the first anneal being performed on the strip prior to any cold rolling step. Three anneals are necessary in the conventional methods in order to reduce, by dissolution and recrystallization, the amount of delta ferrite in the final product to a desired level and to reduce, by homogenization, the amount of dendritic structure. Employing three annealing steps, however, consumes a large amount of time and energy, resulting in a relatively inefficient overall process. Moreover, even after the third anneal, the final strip product often has poor surface quality. Thus, it is desirable to improve the surface quality of the final strip product and to reduce the number of processing steps in the strip casting of stainless steel strip.